A variety of plugs and stoppers have been developed to limit or shut off the flow of liquids from an opening. Many are simply inserted by hand into the opening. This type can be easily removed and reused, but is generally limited to restraining liquids near atmospheric pressure. Other plugs are more forcefully inserted by mechanical means to restrain liquids at higher pressures. This type requires equipment and/or destruction of the plug to remove.
An expandable plug is commonly used for temporary, but reliable sealing of higher pressure fluids. Expandable plugs provide an elastomeric sealing surface which is expanded by compression in an orthogonal direction. Compression is affected by mechanical cams, toggles or linkage assemblies.
These existing temporary plugs require special mechanical components, designed for opening to be stopped. The amount of compression must be adequate to insure reliable sealing under pressure, but excessive force will destroy the elastomer or fluid opening. The mechanical components must also fit into the available space in and near the opening. Space for actuating means must also be provided. Release of existing expandable plugs also requires mechanical force and the associated space and means to apply it. Because of these limitations, each expandable plug only fits a small range of openings. This is especially true of plugs for automatic sprinkler heads.